1. Field of the Invention
The present invention relates to a voltage driving circuit for use in a semiconductor memory device, and more particularly, to a circuit capable of selectively driving different voltages according to different modes of the device.
2. Description of the Related Art
In general, to guarantee the reliability of MOS transistors used in semiconductor memory devices, the power supply voltage of the device is kept as low as possible. Likewise, to operate MOS transistors at high speed, their threshold voltage should also be kept low. In particular, the threshold voltage of MOS transistors should be below 1/4 V.sub.CC to prevent their operating speeds from being too low. For example, when the power supply voltage is 1.5 V, the MOS transistor threshold voltage should be below 0.4 V.
Conversely, to eliminate subthreshold leakage current in a MOS transistor, its threshold voltage should be set above 0.7 V. For example, when a low power supply voltage, i.e., 1.5 V, is used, a leakage current of more than 10.sup.2 -10.sup.5 times that of a MOS transistor using a power supply voltage of 5 V is generated. The leakage current increases the amount of standby current consumed by the high density semiconductor memory device during its standby mode.